On my F150, I monitor; horsepower, intake air temp, average MPG and inst MPG. Definitely has helped my fuel economy in finding how much that extra 5 MPH cost me in MPG. I monitor intake air temp to see how bad my aluminum tubed K & N cold air intake works in stop and go traffic even wrapped in header insulating wrap. Truck is really a dog off the line when intake temps exceed 140. Horsepower is entertaning to see how much is used at idle, uphills, passing and cruising though I'm not sure how accurate that reading is.
Also for those with windshield rear view mirrors, BlendMount has a nice mounting kit.